1. Field of the Invention
The present invention relates in general to a thermal flowmeter including at least one detecting element which utilizes temperature dependence of its electrical resistance to determine a flow rate or velocity of a fluid flowing in a passage. More particularly, the invention is concerned with an improvement on such a thermal flowmeter, which can determine the fluid flow rate with enhanced detecting accuracy.
2. Discussion of the Related Art
As one type of a flowmeter for measuring a flow rate or velocity of a fluid having a varying temperature, e.g., an intake air introduced into an internal combustion engine, there is known a thermal flowmeter as disclosed in laid-open Publication Nos. 55-43447, 56-77716 and 60-91211 of unexamined Japanese Patent Applications, which flowmeter includes detecting elements whose electrical resistance varies with an ambient temperature. This thermal flowmeter is adapted to determine a flow rate of a fluid flowing in a passage, by utilizing the temperature dependence of the electrical resistance of the detecting element.
More specifically, the thermal flowmeter has two detecting elements, each of which includes a substrate made of an electrically insulating material, an electrically resistive body formed on the substrate, and a pair of electrical conductors or leads which are secured to opposite ends of the substrate and electrically connected to the resistor body. The resistor body has an electrical resistance whose value varies with the temperature of the ambient atmosphere. The thermal flowmeter further includes two pairs of electrically conductive supports which are fixed to a wall that defines the fluid passage, and protrude suitable distances from the wall into the passage, for supporting the detecting elements at the electrical conductors.
Upon measurement of the flow rate of the fluid flowing in the fluid passage, one of the two detecting elements is used as a temperature-measuring element which has the same temperature as the fluid to be measured. This detecting element is adapted to measure the temperature of the fluid, by detecting a change in the electrical resistance of the resistive body when it is energized by an electric current which is so small as to generate a negligible amount of heat. The other detecting element is used as a heat-generating element which is energized and heated to a temperature that is higher by a given value than that of the temperature-measuring element. This heat-generating element is adapted to determine the flow rate of the fluid, by detecting the amount of an electric current required to be applied to the heat-generating element for maintaining the above-indicated temperature difference.
In the thermal flowmeter of the above type, a quantity Q of heat which is generated by the heat-generating element is represented by a sum of a quantity Q1 of heat Which is transferred to and dissipated into the fluid flowing in the passage, and a quantity Q2 of heat which is transferred to and dissipated into the wall of the fluid passage, through the electrical conductors and the electrically conductive supports. The quantity Q1 varies with the flow rate of the fluid flowing in the passage, while the quantity Q2 does not vary with the flow rate of the fluid. Accordingly, the amount of the electric current applied to the heat-generating element corresponds to the flow rate of the fluid flowing in the passage, when the current is controlled so that the temperature difference between the heat-generating element and the temperature-measuring element (having the same temperature as the fluid) is kept constant. It follows that the thermal flowmeter is able to determine the flow rate of the fluid, based on the amount of the controlled current applied to the heat-generating element.
Of the quantity Q of the heat generated by the heat-generating element, the quantity Q2 of the heat dissipated into the wall of the fluid passage is determined, depending upon the materials, sizes, or other conditions of the heat-generating element and the conductive supports for supporting the element. In practice, a flowmeter having a specific design, such as installed on automotive internal combustion engines, is often produced in a relatively large lot size, such that the heat-generating element and the conductive supports of each product satisfy the nominal conditions. In such a case, the flow rate of the fluid is calculated based on the amount of the current applied to the heat-generating element, assuming that all of the individual products have the same quantity Q2 of the heat dissipated into the wall of the fluid passage.
However, a study of the inventors of the present invention revealed that a relatively large variation in the quantity Q2 of the heat transferred from the heat-generating element to the wall of the fluid passage exists among the individual products of the flowmeter, even though they are continuously produced under the same condition. Accordingly, the amount of an electric current applied to the heat-generating element for maintaining the temperature difference between the heat-generating and temperature-measuring elements differs from one product to another, even if the flow rate of the fluid is constant. Consequently, the individual flowmeters suffer from a variation in the measurement due to the variation in the quantity Q2 of the heat dissipated into the wall of the fluid passage.
In the thermal flowmeter, both of the electrical conductors of the heat-generating element, and the conductive supports to which the conductors are secured, generally take the form of an electrically conductive thin wire. When the conductors are secured to the conductive supports by welding, for example, it is difficult to accurately position the conductors relative to the corresponding conductive supports, resulting in a variation in the welding positions of the supports among the individual flowmeters. Namely, the individual flowmeters have different conditions of heat transfer between the heat-generating element and the wall of the fluid passage, due to a difference in the distance therebetween, which results from the positioning error. Consequently, there arises a variation in the quantities Q2 of the heat dissipated from the heat-generating elements into the wall of the fluid passage, among the individual flowmeters, whereby the flowmeters suffer from reduction in accuracy in determining the fluid flow rate.
Similarly, the above-described problem of the heat-generating element is encountered by the temperature-measuring element. Namely, the temperature-measuring element also suffers from a variation among the individual flowmeters, in the condition of heat transfer between the element and the wall of the fluid passage, due to the variation in the positional relationship between the conductors and the conductive supports (wall of the fluid passage), which are difficult to be exactly positioned relative to each other when secured to each other. If the wall of the fluid passage has different thermal influences upon the temperature-measuring elements of the individual flowmeters, due to the variation in the condition of heat transfer, the values of the fluid temperature measured by the temperature-measurement elements involve different errors corresponding to the different thermal influences, resulting in reduction in the measuring accuracy of the temperature-measurement element, and eventually in the detecting accuracy of the flowmeter for determining the flow rate of the fluid.